30 



Mr. G. J. Stoney on the Physical [June 20, 



in which the heat is so fierce that carbon, in spite of its want of volatility, 

 and of the enormous pressure to which it is there subjected, boils. The 

 umbra of a spot seems never to form unless when the region in which carbon 

 boils is carried upwards, or the hot region above the clouds is carried down- 

 wards, so as to bring them into contact, and thus entirely obliterate the 

 intervening clouds. It is, however, not safe to attribute to the results 

 stated in this paragraph a probability of more than 1 . 



The trade -winds which blow over the surface of the photosphere are also 

 inquired into. These seem to arise, as Sir John Herschel suspected, from 

 the oblate form of the sun causing a difference in the escape of heat from 

 his poles and equator. There are ascending currents at the poles, descend- 

 ing currents all round the equator. This produces a region of equatorial 

 calms bordered on either side by zones, in the northern of which south-east 

 trades prevail, and in the southern, north-east. These are succeeded by 

 variable winds in the regions of spots, beyond which the polar current blows 

 over the surface of the photosphere in the form of a north-west trade in the 

 northern hemisphere, and a south-west trade in the southern. In the 

 region of spots, both the polar and equatorial currents make their way to 

 a higher level, and in doing so heave up into a colder situation considerable 

 portions of the upper layer of excessively thin cloud, that which is seen only 

 during eclipses. This, though it may at first take place comparatively gently, 

 will be succeeded by a violent upward motion, because the cloud when 

 raised to a cool region will retain a temperature bordering upon that of the 

 photosphere. When this occurs it will both produce the phenomenon of 

 overhanging clouds seen during eclipses, and give rise to a violent cyclone 

 in the regions beneath, immediately over the photosphere. There is no 

 other part of the sun upon which these conditions prevail : hence the limi- 

 tation of spots to two bands parallel to the equator. To these results we 

 may assign the probability 2. 



In the next branch of the inquiry we are obliged to have pretty free re- 

 course to speculation ; and the results, though there is much to be said for 

 them, must be received with the caution which becomes us when we are 

 not at liberty to award a probability higher than 1 . We are forced to 

 invoke an external agent to account for the periodicity of the spots ; and 

 that which is submitted as apparently the most probable, is a swarm of 

 meteors like those which visit the earth in November every thirty-third year, 

 but extended into a much longer stream. These while they pass through 

 the sun's atmosphere would warm the upper regions above his equator, and 

 thus tend to enfeeble the causes which produce the trade-winds. Hence 

 upon each such visit, the trade-winds, the storms which result from them, 

 and the spots which these occasion would all be moderated. It is remark- 

 able that this hypothesis accounts also for the fact that spots prevail more 

 in one hemisphere than the other, inasmuch as the meteors must act more 

 on one hemisphere than the other, and lessen in it the causes that produce 

 spots, unless we make the highly improbable supposition that the axis 



